Re: normalize audio in mp4s
On Wed 11 Mar 2020 at 19:33:59 (+0300), Reco wrote:
> On Wed, Mar 11, 2020 at 05:00:21PM +0100, Emanuel Berg wrote:
> > Stream #0:1(por): Audio: ac3, 48000 Hz, 5.1(side), fltp, 448 kb/s (default)
>
> Or this, dependent on your preferred language
>
> > Stream #0:2(eng): Audio: ac3, 48000 Hz, 5.1(side), fltp, 448 kb/s
>
> Is the likely source of your problem.
>
> This particular video has 5.1 audio, and you most probably have only two
> audio channels to play it.
>
> If you're using mpv, you can try adding --audio-channels=stereo, but
> (YMMV of course) I find its downmix lacking.
>
> Personally I just reencode audio in such cases.
> For instance (0:1 is the first audio track, use 0:2 for english):
>
> ffmpeg -i ford-v-ferrari-2019.mkv -map 0:0 -map 0:1 \
> -c:v copy \
> -c:a libopus -b:a 128k -ac 2 -clev 1.414 -slev .5 \
> ford-v-ferrari-2019.good_audio.mkv
That's a useful little recipe, and pleased the OP, but can you just
clarify the arguments -clev 1.414 -slev .5
I get the impression that for resampling, the number should be in
dB, whereas for downmixing (-center_mixlev and -surround_mixlev),
they're scale factors, which is what yours look like).
And how do you decide to resample rather than downmix? (being
clueless at this stuff).
Cheers,
David.
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